Current-driven light-emitting display apparatus and method of producing the same

ABSTRACT

A method for producing an organic EL display device is disclosed. The display device comprises a substrate, a transistor disposed on the substrate, a flattened inter-layer insulation film covering the transistor, a pixel electrode, and an organic EL layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of Invention

[0002] The invention relates to a display apparatus in which acurrent-driven light-emitting device such as an organic electroluminescence (hereinafter referred to a “EL”) display device, are drivenby using thin-film transistors. More particularly, the invention relatesto a current-driven light-emitting display apparatus driven bythin-film-transistors, which realizes the suppression of deteriorationwith time, and to a method of producing the same.

[0003] 2. Description of Related Art

[0004] The inventor of this invention carefully examined organic ELdisplay devices driven by thin-film transistors, and ascertained thefollowing facts.

[0005] (1) In an organic EL display device driven by thin-filmtransistors, since the organic EL display device is a direct-currentdevice, direct current also runs through thin-film transistors, whichare connected in series to the EL device, for the purpose of controllingit.

[0006] (2) Thin-film transistors are classified into an n-channel typeand a p-channel type. These types differ extremely in the manner inwhich deterioration with time occurs.

[0007] Accordingly, an object of the present invention is to suppressthe deterioration with time of thin-film transistors in a currentluminescent device driven by the thin-film transistors.

SUMMARY OF THE INVENTION

[0008] (1) In the present invention, there is provided a current-drivenlight-emitting display apparatus comprising a plurality of scanninglines and a plurality of data lines, thin-film transistors and currentluminescent devices being formed in positions corresponding to each ofthe intersections of the scanning lines and the data lines, wherein atleast one of the thin-film transistors is a p-channel type thin-filmtransistor.

[0009] It is possible to suppress the deterioration with time of athin-film transistor with this apparatus.

[0010] (2) In the present invention, there is provided a current-drivenlight-emitting display apparatus in which a plurality of scanning linesa plurality of data lines, common electrodes, and opposite electrodesare formed, with first thin-film transistors being formed in positionscorresponding to the intersections of the scanning lines and the datalines, second thin-film transistors, holding capacitors, pixelelectrodes, and current luminescent elements, the first thin-filmtransistors controlling conductivity between the data lines and theholding capacitors by the potentials of the scanning lines, the secondthin-film transistors controlling conductivity between the commonelectrodes and the pixel electrodes by the potentials of the holdingcapacitors, to thereby control the current which flows through thecurrent luminescent elements provided between the pixel electrodes andthe opposite electrodes wherein the second thin-film transistors arep-channel type thin-film transistors.

[0011] (3) In the present invention, there is provided a current-drivenlight-emitting display apparatus according to (1) or (2), furthercomprising a driving circuit for driving the current luminescentelement, the driving circuit is comprised of the plurality of scanninglines, the plurality of data lines, the thin-film transistors, and thecurrent luminescent elements, which are disposed on the substrate,wherein the p-channel type thin-film transistors are formed in the samestep as the thin-film transistors in the driving circuits.

[0012] (4) In the current-driven light-emitting display apparatusaccording to any of (1) or (3), the thin-film transistors arepolysilicon thin-film transistors.

[0013] (5) The invention provides a current-driven light-emittingdisplay apparatus according to (3), wherein the drive circuits comprisecomplementary type thin-film transistors, the first thin-filmtransistors are formed in the same step as n-channel type thin-filmtransistors in the driving circuits, and the second thin-filmtransistors are formed in the same step as the p-channel type thin-filmtransistors in the driving circuits.

[0014] According to (5), it is possible to provide a current-drivenlight-emitting display apparatus, which exhibits high performance withno deterioration with time, without increasing the number of steps forproducing the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a block diagram of the basic structure of a display towhich the present invention is applied;

[0016]FIG. 2 is an equivalent circuit diagram of a display deviceequipped with thin-film transistors according to a first embodiment ofthe present invention;

[0017]FIG. 3 is a drive voltage diagram of the display device equippedwith thin-film transistors according to the first embodiment of thepresent invention;

[0018]FIG. 4 is a current-voltage characteristic chart of acurrent-thin-film transistor according to the first embodiment of thepresent invention;

[0019]FIG. 5 is a current-voltage characteristic chart of an organic ELdisplay device according to the first embodiment of the presentinvention;

[0020]FIG. 6(a) is a sectional view of an organic display EL deviceequipped with thin-film transistors according to the first embodiment ofthe invention, and FIG. 6(b) is a plan view of an organic display ELdevice according to the first embodiment of the present invention;

[0021]FIG. 7 is an equivalent circuit diagram of an organic EL displaydevice equipped with thin-film transistors used in a second embodimentof the present invention;

[0022]FIG. 8 is a drive-voltage diagram of an organic EL display deviceequipped with thin-film transistors according to the second embodimentof the present invention;

[0023]FIG. 9 is a current-voltage characteristic chart of acurrent-thin-film transistor according to the second embodiment of thepresent invention;

[0024]FIG. 10 is a current-voltage characteristic chart of an organic ELdisplay device according to the second embodiment of the presentinvention;

[0025]FIG. 11(a) is a sectional view of an organic EL display deviceequipped with thin-film transistors according to the second embodimentof the present invention, and FIG. 11(b) is a plan view of an organic ELdisplay device equipped with thin-film transistors according to thesecond embodiment of the present invention;

[0026]FIG. 12 is a chart showing the deterioration with time of ann-channel type thin-film transistor;

[0027]FIG. 13 is a chart showing the deterioration with time of ap-channel type thin-film transistor; and

[0028] FIGS. 14(a)-(d) are flow diagrams of the process for producing athin-film-transistor-drive organic EL display device according to thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0029] (The General Structure of an Organic El Display Device)

[0030] Referring to the drawings, the preferred embodiments of thepresent invention will be described.

[0031] As shown in FIG. 1, the center region of a substrate 1constitutes a display part. In the outer periphery of the transparentsubstrate 1, at the top side of the drawing, a data-side drive circuit3, which outputs image signals to data lines 112, is arranged, and atthe left side of the drawing, a scanning-side drive circuit 4, whichoutputs scanning signals to scanning lines 111, is arranged. In thesedrive circuits 3 and 4, n-type thin-film transistors and p-typethin-film transistors form complementary type TFTs. These complementarytype thin-film transistors are included in shift register circuits,level shift circuits, analog switch circuits, etc.

[0032] Arranged on the transparent substrate 1 are a plurality ofscanning lines 111, and a plurality of data lines 112 extending in adirection perpendicular to the direction in which the scanning linesextend. The intersections of these data lines 112 and scanning lines 111constitute pixels 7 in the form of a matrix.

[0033] Formed in each of these pixels 7 is a first thin-film transistor(hereinafter referred to as “a switching thin-film transistor”) 121, inwhich scanning signals are supplied to a gate electrode 21 (a first gateelectrode) through the scanning line 111. One end of the source/drainregion of the switching thin-film transistor 121 is electricallyconnected to a data line 112, while the other end of the source/drainregion is electrically connected to a potential holding electrode 113.In addition, a common line 114 is disposed in parallel to the scanningline 111. Holding capacitor 123 is formed between the common line 114and the potential holding electrode 113. The common line is maintainedat a controlled potential. Accordingly, when the switching thin-filmtransistor 121 is turned ON through the selection by a scanning signal,the image signal from the data line 112 is written to the holdingcapacitor 123 through the switching thin-film transistor.

[0034] The potential holding electrode 113 is electrically connected tothe gate electrode of second thin-film transistor 122 (hereinafterreferred to as “a current-thin-film transistor”). The one end of thesource/drain region of the current-thin-film transistor 122 iselectrically connected to a common line 114, while, the other end of thesource/drain region is electrically connected to one electrode 115 of aluminescent element 131. When the current-thin-film transistor 122 isturned ON, the current of the common line 114 flows to the luminescentelement 131 of such as an organic EL display device through thecurrent-thin-film transistor 122, so that the luminescent element 131emits light. Further, although one electrode of the holding capacitor isconnected to a common line 114 in this arrangement, it is also possiblefor it to be connected to a capacitance line being provided separately,instead of being connected to the common line 114. Alternatively, oneelectrode of the holding capacitor may be connected to an adjacent gateline.

[0035] (First Embodiment)

[0036]FIG. 2 is a block diagram of an organic EL display device equippedwith thin-film transistors, according to a first embodiment of thepresent invention. FIG. 3 is a drive voltage diagram of an organic ELdisplay device with thin-film transistors, according to the firstembodiment of the present invention. FIG. 4 is a current-voltagecharacteristic diagram of a current-thin-film transistor according tothe first embodiment of the present invention. FIG. 5 is acurrent-voltage characteristic chart of an organic EL display device,according to the first embodiment of the present invention.

[0037] In FIG. 2, there are shown a scanning line 111, a data line 112,a holding electrode 113, a common line 114, a pixel electrode formed ofAl 115, an opposite electrode formed of ITO 116, a switching thin-filmtransistor 121, an n-channel type current-thin-film transistor 122, aholding capacitor 123, an organic EL display element 131 (hereinafterreferred to as “a forward oriented organic EL display device”) which iscaused to emit light by the current flowing to the pixel electrode 115from the opposite electrode 116, and the current directions of theorganic EL display device 131 and 141.

[0038] In FIG. 3, there are shown a scanning potential 211, a signalpotential 212, a holding potential 213, a common potential 214, a pixelpotential 215, and a counter potential 216. FIG. 3, only a part of eachpotential is shown to illustrate the respective potential relationships.The potential of the scanning line 111 corresponds to the scanningpotential 211; the potential of the data line 112 corresponds to thesignal potential 212; the potential of the holding electrode 113corresponds to the holding potential 213; the potential of the commonline 114 corresponds to the common potential 214; the potential of thepixel electrode 115 formed of Al corresponds to the pixel potential 215;and the potential of the opposite electrode 116 formed of ITO (IndiumTin oxide) corresponds to the counter potential 216. FIG. 3 shows eachsignal potential schematically and partially.

[0039] Numeral 221 indicates a period in which a pixel is in thedisplay-state, wherein current flows into the forward oriented organicEL display element 131, so that it emits light, and numeral 222indicates a period in which the pixel is in the non-display state,wherein current does not flow into the forward oriented organic ELdisplay element 131, so that it does not emit light.

[0040] Referring to FIG. 4, a curve 31 indicates the current-voltagecharacteristic of the n-channel type current-thin-film transistor 122 asobserved when the drain voltage is 4V, and a curve 32 indicates thecurrent-voltage characteristic of the n-channel type current-thin-filmtransistor 122 as observed when the drain voltage is 8V. Regardingeither drain voltage, the following facts can been seen. When the gatevoltage is low, the n-channel type current-thin-film transistor 122 isturned OFF and a small amount of drain current flows demonstrating ahigh source/drain resistance. When the gate voltage is high, then-channel type current-thin-film transistor 122 is turned ON and a largeamount of drain current flows demonstrating a low source/drainresistance.

[0041] In FIG. 5, numeral 4 indicates the current-voltage characteristicof the forward oriented organic EL display element 131. Here, thevoltage represents the counter potential 216 against the pixel potential215, and the current represents the current which flows to the pixelelectrode 115 from the opposite electrode 116. The forward orientedorganic EL display element 131 is OFF when the voltage is not higherthan a certain threshold voltage; the resistance is high and allows nocurrent to flow, so that the device does not emit light. The device isON when the voltage is over a certain threshold voltage, and theresistance is low and allows current to flow, so that the device emitslight. In this case, the threshold voltage is approximately 2V.

[0042] The operation of an organic EL display device equipped with thethin-film transistors of this embodiment will be described withreference to FIG. 2, FIG. 3, FIG. 4, and FIG. 5.

[0043] The switching thin-film transistor 121 controls the conductivitybetween the data line 112 and the holding electrode 113 by means of thepotential of the scanning line 111. In other words, the scanningpotential 211 controls the conductivity between the signal potential 212and the holding potential 213. While in this example, the switchingthin-film transistor 121 is an n-channel type thin-film transistor, ap-channel type thin-film transistor is also applicable.

[0044] For the period 221 in which the pixel is in the display-state,the signal potential 212 is high, and the holding potential 213 isretained at a high level. For the period 222 in which the pixel is inthe non-display state, the signal potential 212 is low, and the holdingpotential 213 is retained at a low level.

[0045] The n-channel type current-thin-film transistor 122 has thecharacteristic as shown in FIG. 3 and controls the conductivity betweenthe common line 114 and the pixel electrode 115 by means of thepotential of the holding electrode 113. In other words, the holdingpotential 213 controls the conductivity between the common potential 214and the pixel potential 222. For the period 221 in which the pixel is inthe display-state, the holding potential 213 is high, so that the commonline 114 is electrically connected to the pixel electrode 115. For theperiod 222 in which the pixel is in the non-display state, the holdingpotential 213 is low, so that the common line 114 is disconnected fromthe pixel electrode 115.

[0046] The organic EL display element 131 has the characteristic asshown in FIG. 5. For the period 221 in which the pixel is in thedisplay-state, the current flows between the pixel electrode 115 and theopposite electrode 116, so that the organic EL display element 131 emitslight. For the period 222 in which the pixel is in the non-displaystate, no current flows, so that the device does not emit light.

[0047]FIG. 6(a) is a sectional view of a thin-film transistor organic ELdisplay device (1 pixel) according to an embodiment of the presentinvention. FIG. 6(b) is a plan view of a thin-film transistor organic ELdisplay device (1 pixel) according to an embodiment of the presentinvention. The section taken along the line A-A′ of FIG. 6(a)corresponds to the section taken along the line A-A′ of FIG. 6(b).

[0048] In FIG. 6(a), numeral 132 indicates a hole injection layer,numeral 133 indicates an organic EL layer, and numeral 151 indicates aresist.

[0049] In this example, the switching thin-film transistor 121 and then-channel type current-thin-film transistor 122 adopt the structure andthe process ordinarily used for a low-temperature polysilicon thin-filmtransistor, which are used for thin-film transistor liquid crystaldisplay devices, i.e., a top-gate structure and a process conducted inthe condition that the maximum temperature is 600° C. or less. However,other structures and processes are also applicable.

[0050] The forward oriented organic EL display element 131 is formed bythe pixel electrode 115 formed of Al, the opposite electrode 116 formedof ITO, the hole injection layer 132, and the organic EL layer 133. Inthe forward oriented organic EL display element 131, the direction ofcurrent of the organic EL display device, indicated at 141, can be setfrom the opposite electrode 116 formed of ITO to the pixel electrode 115formed of Al. Further, the structure of the organic EL display device isnot restricted to the one used here. Other structures are alsoapplicable, as long as the direction of current of the organic ELdisplay device, indicated at 141, can be set to the direction from theopposite electrode to pixel electrode.

[0051] Here, the hole injection layer 132 and the organic EL layer 133are formed by an ink-jet printing method, employing the resist 151 as aseparating structure between the pixels, and the opposite electrode 116formed of ITO is formed by a sputtering method, yet other methods arealso applicable.

[0052] In this embodiment, the common potential 214 is lower than thecounter potential 216, and the current-thin-film transistor is then-channel type current-thin-film transistor 122.

[0053] In the period 221 in which the pixel is in the display-state, then-channel type current-thin-film transistor 122 is ON. The current whichflows through the forward oriented organic EL display element 131, i.e.,the ON-current of the n-channel type current-thin-film transistor 122depends on the gate voltage, as shown in FIG. 4. Here, the term “gatevoltage” means the potential difference between the holding potential213 and the lower one of the common potential 214 and the pixelpotential 215. In this embodiment, the common potential 214 is lowerthan the pixel potential 215, so that the gate voltage indicates thepotential difference between the holding potential 213 and the commonpotential 214. The potential difference can be sufficiently large, sothat a sufficiently large amount of ON-current is obtainable. TheON-current of the n-channel type current-thin-film transistor 122 alsodepends on the drain voltage. However, this does not affect the abovesituation.

[0054] Conversely, in order to obtain a necessary amount of ON-current,the holding potential 213 can be made lower, and the amplitude of thesignal potential 212 and therefore the amplitude of the scanningpotential 211 can be decreased. In other words, in the switchingthin-film transistor 121 and the n-channel type current-thin-filmtransistor 122, a decrease in drive voltage can be achieved withoutentailing any loss in image quality, abnormal operations, or a decreasein the frequency enabling them to operate.

[0055] Further, in the embodiment of the present invention, the signalpotential 212 for the pixel to be in the display-state is lower than thecounter potential 216.

[0056] As stated above, in the period 221 in which the pixel is in thedisplay-state, the ON-current of the n-channel type current-thin-filmtransistor 122 depends on the potential difference between the holdingpotential 213 and the common potential 214, but not directly on thepotential difference between the holding potential 213 and the counterpotential 216. Thus, the holding potential 213, i.e., the signalpotential 212 for the pixel to be in the display-state, can be madelower than the counter potential 216, and therefore, the amplitude ofthe signal potential 212 and the amplitude of the scanning potential 211can be decreased, while retaining a sufficiently large ON-current in then-channel type current-thin-film transistor 122. That is, in theswitching thin-film transistor 121 and the n-channel typecurrent-thin-film transistor 122, a decrease in drive voltage can beaccomplished without entailing any loss in image quality, abnormaloperations, and a decrease in the frequency enabling them to operate.

[0057] Moreover, in this embodiment, the signal potential 212 for thepixel to be in the non-display-state is higher than the common potential214.

[0058] In the period 222 in which the pixel is in the non-display-state,when the signal potential 212 becomes slightly higher than the commonpotential 214, the n-channel type current-thin-film transistor 122 isnot completely turned OFF. However, the source/drain resistance of then-channel type current-thin-film transistor 122 becomes considerablyhigher, as shown in FIG. 4. Thus, the pixel potential 215, which isdetermined by dividing the common potential 214 and the counterpotential 216 by the values of the resistance of the n-channel typecurrent-thin-film transistor 122 and the resistance of the forwardoriented organic EL display element 131, becomes a potential close tothe counter potential 216.

[0059] The voltage which is applied to the forward oriented organic ELdisplay element 131 is the potential difference between the pixelpotential 215 and the counter potential 216. As shown in FIG. 5, theforward oriented organic EL display element 131 is turned OFF when thevoltage is not higher than a certain threshold voltage, when no currentflows, so that the display device does not emit light. Namely, theutilization of a threshold potential of the forward oriented organic ELdisplay element 131 makes it possible for the forward oriented organicEL display element 131 not to emit light, even if the signal potential212 is slightly higher than the common potential 214, and the n-channeltype current-thin-film transistor 122 is not completely turned OFF.

[0060] Here, the amplitude of the signal potential 212, and thereforethe amplitude of the scanning potential 211 can be decreased by makingthe signal potential 212 for the pixel to be in the non-display state tobe higher than the common potential 214. In other words, with regard tothe switching thin-film transistor 121 and the n-channel typecurrent-thin-film transistor 122, a decrease in drive potential can beaccomplished without entailing any loss of image quality, abnormaloperations, or a decrease in the frequency enabling them to operate.

[0061] The operation of an organic EL display device equipped with thethin-film transistors of this embodiment is not as simple as describedabove; it operates under a more complicated relationship between voltageand current. However, the description above holds true approximately andqualitatively.

[0062] (Second Embodiment)

[0063]FIG. 7 is an equivalent circuit diagram of an organic EL displaydevice equipped with thin-film transistors, according to the secondembodiment of the present invention. FIG. 8 is a drive voltage diagramof the organic EL display device with thin-film transistors, accordingto the second embodiment of the present invention. FIG. 9 is acurrent-voltage characteristic chart of the organic EL display deviceaccording to the second embodiment of the present invention.

[0064] In FIG. 7, there are shown a pixel electrode formed of ITO 615,an opposite electrode formed of Al 616, a p-channel typecurrent-thin-film transistor 622, and an organic EL display device 631(hereinafter referred to as “a reverse oriented organic EL displaydevice”), which is caused to emit light by the current flowing to theopposite electrode 616 from the pixel electrode 615. Numeral 641indicates the direction of the current of the organic EL display device.This direction is the reverse of that shown in FIG. 2. Except for this,this embodiment is the same as the above first embodiment shown in FIG.2.

[0065]FIG. 8 is the same as FIG. 3 except that the level of eachpotential is different from that of FIG. 3.

[0066] In FIG. 9, a curve 81 indicates a current-voltage characteristicof a p-channel type current-thin-film transistor 622 as observed whenthe drain voltage is 4V. A curve 82 indicates a current-voltagecharacteristic of the p-channel type current-thin-film transistor 622 asobserved when the drain voltage is 8V.

[0067] In FIG. 10, a curve 9 indicates a current-voltage characteristicof a reverse oriented organic EL display device 631.

[0068] The organic EL display device equipped with the thin-filmtransistors of this embodiment operates in the same way as that of thefirst embodiment, except that the potential relationship regarding thecurrent-thin-film transistor is reversed due to the fact that thecurrent-thin-film transistor is the p-channel type thin-film transistor622.

[0069]FIG. 11(a) is a sectional view of an organic EL display device (1pixel) equipped with the thin-film transistors, according to the secondembodiment of the present invention. FIG. 11(b) is a plan view of athin-film transistor organic EL display device (1 pixel), according tothe second embodiment of the present invention. The section taken alongthe line A-A′ of FIG. 11(a) corresponds to the section taken along theline A-A′ of FIG. 11(b).

[0070]FIG. 11(a) is the same as FIG. 6(a), except that it shows a holeinjection layer 632 and an organic EL layer 633.

[0071] The reverse oriented organic EL display device 631 is formed bymeans of the pixel electrode 615 formed of ITO, the opposite electrode616 formed of Al, the hole injection layer 632, and the organic EL layer633. In the reverse oriented organic EL display device 631, thedirection of current of the organic EL display device, indicated at 641,can be set to the direction from the pixel electrode 615 formed of ITOto the opposite electrode 616 formed of Al.

[0072] In this embodiment, a common potential 714 is higher than acounter potential 716. Further, the current-thin-film transistor is thep-channel type current-thin-film transistor 622.

[0073] In this embodiment, a signal potential 712 for the pixel to be inthe display-state is higher than the counter potential 716.

[0074] Furthermore, in this embodiment, the signal potential 712 for thepixel to be in the non-display-state is lower than the common potential714.

[0075] All of the effects of the thin-film transistor organic EL displaydevice of this embodiment are also the same as those of the firstembodiment, except that the potential relationship regarding thecurrent-thin-film transistor is reversed due to the fact that thecurrent-thin-film transistor is the p-channel type thin-film transistor622.

[0076] In this embodiment, the current-thin-film transistor 122 is ap-channel type thin-film transistor. This arrangement enables thedeterioration with time of the current-thin-film transistor 122 tosignificantly decrease. Furthermore, the arrangement adopting ap-channel type polysilicon thin-film transistor enables thedeterioration with time of the current-thin-film transistor 122 todecrease even further.

[0077]FIG. 14 is a diagram of a process of producing the current-drivenlight-emitting display apparatus equipped with the thin-filmtransistors, according to the embodiment of the present inventiondescribed above.

[0078] As shown in FIG. 14(a), an amorphous silicon layer with athickness of 200 to 600 angstroms is deposited all over a substrate 1,and the amorphous silicon layer is poly-crystallized by laser annealingetc., to form a polycrystalline silicon layer. After this, patterning isperformed on the polycrystalline silicon layer to form a siliconthin-film 421, which serves as a source/drain channel region of theswitching thin-film transistor 121, a first electrode 423 of the storagecapacitor 123, and a silicon thin-film 422, which serves as asource/drain channel region of the current-thin-film transistor 122.Next, an insulation film 424, which serves as a gate insulation film, isformed over the silicon thin-films 421, 422, and the first electrode423. Then, implantation of phosphorous (P) ions is selectively effectedon the first electrode 423 to lower the resistance thereof. Next, asshown in FIG. 14(b), gate electrodes 111 and 111′, which consist of TaNlayers, are formed on the silicon thin-films 421 and 422 through theintermediation of the gate insulation film. Next, a resist mask 42 isformed on the silicon layer 422 serving as a current-thin-filmtransistor, and phosphorous (P) ions are implanted throughself-alignment using the gate electrode as a mask to form an n-typesource/drain region in the silicon layer 421. Subsequently, as shown inFIG. 14(c), a resist mask 412′ is formed on the first silicon layer 421and the first electrode, and boron (B) is ion-implanted in the siliconlayer 422 through self-alignment using the gate electrode 111′ as a maskto form a p-type source/drain region in the silicon layer 422. In thisway, an n-channel type impurity doping 411 allows the switchingthin-film transistor 121 to be formed. At this time, thecurrent-thin-film transistor 122 is protected by the resist mask 42, sothat the n-channel type impurity doping 411 is not performed. Then, ap-channel type impurity doping 412 allows the current-thin-filmtransistor 122 to be formed.

[0079] Further, though not illustrated, in a case in which a shiftregister of a drive circuit section which drives the switchingtransistor 121, and a thin-film transistor constituting a sample holdcircuit etc., are to be formed on the same substrate, it is possible toform them simultaneously in the same step of process as has beendescribed above.

[0080] A second electrode 425 of the storage capacitor may be formedtogether with the gate electrodes 111 and 111′ simultaneously, either ofthe same or different materials.

[0081] As shown in FIG. 14(d), after the formation of an inter-layerinsulation film 43 and, then, contact holes, electrode layers 426, 427,428 and 429 formed of aluminum, ITO or the like are formed.

[0082] Next, after an inter-layer insulation film 44 is formed andflattened, contact holes are formed; then, ITO 45 is formed with athickness of 1000 to 2000 angstroms, preferably about 1600 angstroms, insuch a manner that one electrode of the current-thin-film transistor isconnected thereto. For each pixel region, bank layers 46 and 47, whichare not less than 2.0 μm in width, are defined. Next, an organic ELlayer 48 is formed by an ink-jet method etc., in the region surroundedby the bank layers 46 and 47. After the organic EL layer 48 is formed,an aluminum-lithium layer with a thickness of 6000 to 8000 angstroms isdeposited as an opposite electrode 49 on the organic EL layer 48.Between the organic EL layer 48 and the opposite electrode 49, a holeinjection layer may be disposed, as shown in FIG. 6(a).

[0083] The process mentioned above enables an organic EL display devicedriven by means of a high-performance thin-film transistor to be formed.Since polysilicon is much higher in the mobility of carriers thanamorphous-silicon, a rapid operation is possible.

[0084] In particular, in this embodiment, when the p-typecurrent-thin-film transistor 122 and the n-type switching thin-filmtransistor 121 are formed, it is possible to form both of n-type andp-type thin-film transistors, which are complementary type thin-filmtransistors constituting a shift register of a drive circuit, a samplehold circuit and the like, being simultaneously formed in the abovementioned embodiment. The arrangement makes it possible to realize aconstruction capable of decreasing the deterioration with time of thecurrent-thin-film transistor 122, without increasing the number ofproduction steps.

[0085] As described above, in the first embodiment, an n-channel typecurrent-thin-film transistor is used, and, in the second embodiment, ap-channel type current-thin-film transistor is used. Here, thedeterioration with time of p-channel and n-channel type thin-filmtransistors will be examined.

[0086]FIG. 12 and FIG. 13 are charts showing respectively thedeterioration with time of n-channel type and p-channel type thin-filmtransistors, especially of polysilicon thin-film transistors, underequivalent voltage application conditions. Numerals 511 and 512 of FIG.12 indicate the conductivity characteristics of an n-channel typethin-film transistor, in the cases in which Vd=4V and in which Vd=8V,respectively, before voltage application. Numerals 521 and 522 indicatethe conductivity characteristics of an n-channel type thin-filmtransistor, in the cases in which Vg=0V and Vd=15V and in which Vd=4Vand Vd=8V, respectively, after voltage application of approximately 1000seconds. Numerals 811 and 812 of FIG. 13 indicate the conductivitycharacteristics of a p-channel type thin-film transistor in the cases inwhich Vd=4V, and in which Vd=8V, respectively, before voltageapplication. Numerals 821 and 822 indicate the conductivitycharacteristics of a p-channel type thin-film transistor in the cases inwhich Vg=0V and Vd=15V, and in which Vd=4V and Vd=8V, respectively,after voltage application for approximately 1000 seconds. It can be seenthat in the p-channel type thin-film transistor, the decrease ofON-current and the increase of OFF-current are smaller than in then-channel type.

[0087] Taking into consideration the difference in thedeterioration-with-time characteristic between the p-type and the n-typethin-film transistors as shown in FIG. 12 and FIG. 13 respectively, atleast either a switching thin-film transistor or a current-thin-filmtransistor is formed of a p-channel type thin-film transistor,especially a p-type polysilicon thin-film transistor, whereby thedeterioration with time can be suppressed. Further, by forming theswitching thin-film transistor as well as the current-thin-filmtransistor of a p-type thin-film transistor, it is possible to maintainthe characteristics of the display device.

[0088] While an organic EL display device is used as the luminescentdevice in the embodiment described above, this should not be construedrestrictively, yet, it is needless to say that an inorganic EL displaydevice or other current-driven luminescent devices are also applicable.

[0089] Industrial Applicability

[0090] The display apparatus according to the present invention can beused as a display apparatus equipped with a current-driven luminescentdevice such as an organic EL display device or an inorganic EL displaydevice, and a switching device to drive the luminescent device, such asa thin-film transistor.

What is claimed is:
 1. A method for producing a display, the stepscomprising: forming a transistor on a substrate; forming and flatteningan inter-layer insulation film over the transistor, forming a contacthole in the inter-layer insulation film; forming a pixel electrode onthe inter-layer insulation film; and forming an organic EL layer in apixel region corresponding to the pixel electrode, the transistor beingconnected to the pixel electrode through the contact hole.
 2. A methodfor producing a display, the steps comprising: forming a transistor on asubstrate; forming a first inter-layer insulation film over thetransistor; forming and flattening a second inter-layer insulation filmover the first inter-layer insulation film; forming a pixel electrode onthe flattened second inter-layer insulation film; forming an organic ELlayer in a pixel region corresponding to the pixel electrode; andforming a contact hole in the first inter-layer insulation film and thesecond inter-layer insulation film, the transistor being connected tothe pixel electrode through the contact hole.
 3. A method for producinga display, the steps comprising: forming a transistor on a substrate;forming a first inter-layer insulation film over the transistor; forminga first contact hole in the first inter-layer insulation film, thetransistor being connected to an electrode layer through the firstcontact hole; forming and flattening a second inter-layer insulationfilm over the first inter-layer insulation film and the electrode layer;forming a pixel electrode on the second inter-layer insulation filmafter the flattening process of the second inter-layer insulation film;forming an organic EL layer in a pixel region corresponding to the pixelelectrode; and forming a second contact hole in the second inter-layerinsulation film, the transistor being connected to the pixel electrodethrough the second contact hole.
 4. The method according to claim 1,further comprising forming a bank layer for defining the pixel region.5. The method according to claim 1, the forming of the organic EL layerbeing performed by an ink-jet method.
 6. A display, comprising: asubstrate; a transistor disposed on the substrate; a flattenedinter-layer insulation film covering the transistor; a pixel electrodeformed on the flattened inter-layer insulation film; and an organic ELlayer disposed between the pixel electrode and a counter electrode. 7.The display according to claim 6, further comprising a contact hole, thetransistor being connected to the pixel electrode through the contacthole.
 8. The display according to claim 6, the organic EL layer beingsurrounded by a bank layer.
 9. The display according to claim 8, thebank layer being formed on the inter-layer insulation film.